1. Field Of The Invention
The present invention relates to an amplifier, and more particularly, to an amplifier having a plurality of differential amplifier circuits configured for improving the distortion ratio and other amplifier characteristics.
2. Description of the Prior Art
As shown in FIG. 1, a typical prior-art amplifier 10 includes a transistor Qa which receives, at its base, an input signal voltage from a signal source 12. The transistor Qa is connected at its emitter to ground through a resistor Re and an amplified output signal is produced at its collector.
In general, the base-emitter voltage versus base current (collector current) characteristic of a transistor is exponential. Therefore, in the case of the amplifier shown in FIG. 1 when the input signal is large in amplitude, the distortion of the output current becomes large. In order to expand the input dynamic range and thereby reduce the distortion, the amplifier input may be connected with the resistor Re for establishing a current negative feedback path. However, when the output impedance Rg of the signal source 12 is low and the resistor Re is connected to the amplifier input, the noise (thermal disturbance noise) generated from the resistor Re causes the noise factor NF to deteriorate. Further, because a signal voltage is produced across the resistor Re, the DC voltage at the emitter of the transistor Qa must be correspondingly increased. Thus, the power source voltage has to be increased. As a result, the prior-art amplifier is not suitable for use with an integrated circuit (IC) for a radio receiver driven by a single size AA battery.
In order to reduce the noise generated from the resistor Re, a smaller resistance value for the resistor Re may be used, and, as a result thereof, the signal voltage decreases. However, if the resistance value of the resistor Re is reduced, the amount of negative feedback current is also undesirably reduced. For avoiding a reduction of the negative feedback current, the collector current of the transistor Qa may be increased, but that causes an undesirable increase in the power consumption.
The distortion may be minimized by selecting the resistance value of the resistor Re and the operation point of the transistor Qa so that the resistance value of the resistor Re is 1/2 the emitter resistance value Re of the transistor Qa. However, the operation point of transistor Qa will fluctuate when the input signal voltage is large, thereby precluding direct connection of the transistor Qa to a transistor at a next stage.
As a result of the above-described difficulties, the prior-art amplifier is not suitable for fabrication as an integrated circuit.